Volatile organic compounds such as hydrocarbons (VOCs) pose serious soil and water contamination problems from spills, leakage from storage facilities, or surface discharges. These contaminants can contribute to ground water degradation and can pose a health threat to drinking water supplies. The contaminants can also remain within the vadose zone. Among the VOC contaminants frequently found in the vadose zone and illustrative of the problem are those used in paint manufacture (acetone, methylisobutylketone, methylethylketone, toluene, xylene, mineral spirits, naphtha), halogenated hydrocarbons such as trichloroethylene and perchloroethylene, which are common solvent and cleaning agents, hydrocarbons from petroleum processing or transport, among many others.
A variety of methods have been proposed for recovery of volatile contaminants from the vadose zone. Thus, Isser et al., U.S. Pat. No. 4,593,760, issued Jun. 10, 1986, proposes the use of vacuum extraction. Payne, U.S. Pat. No. 4,730,672, issued Mar. 15, 1988, and Bernhardt et al., U.S. Pat. No. 4,886,119, issued Dec. 12, 1989, also disclose methods and apparatus for driving and collecting volatile contaminants from contaminated soils.
Contaminants are often recovered as both a liquid stream and as gases. Hess et al., U.S. Pat. No. 5,050,676, issued Sep. 24, 1991, Morrow, U.S. Pat. No. 5,076,360, issued Dec. 31, 1991, and Hajali, U.S. Pat. No. 5,172,764, issued Dec. 22, 1992, all address removing contaminants as separate liquid and gas streams.
However, contaminant removal addresses only the first part of the problem. Once removed, disposal (that is, destruction or conversion to non-toxic or less toxic species) requires attention.
High energy electron accelerators have received considerable interest for treating contaminated water. One research group at the University of Florida has studied use of high energy electrons for treating various toxic and hazardous organic chemicals in aqueous solutions. Lawrence Livermore National Laboratory and collaborators have been investigating the decomposition of chlorinated hydrocarbons using x-rays; they have demonstrated the destruction of volatile organic compounds in ground water and for both polychlorinated biphenyls and insecticides in organic solutions. Gehringer et al., Appl. Radiat. Isot., 43 (9), pp. 1107-1115 (1992), discusses the removal of chlorinated ethylenes, such as trichloroethylene and perchloroethylene, in ground water by ozone-electron beam irradiation treatment. However, the destruction of contaminants in water is distinctly different from the problems encountered with contaminants in gas or vapor.
Electron-beam technology has also been suggested for destroying low concentrations of vinyl chloride in carrier gases. Slater et al., J. Appl. Phys., 52 (9), pp. 5820-5828 (1981). However, the chemistry of electron-beam exposure of air is very complex owing to the high energy of the primary electrons and the large number of ionization events created by each primary electron. Electron beam exposure of air probably forms a wide spectrum of excited oxygen and nitrogen species of which some are HO, NO, NO.sub.2, O.sub.3, and O.sub.2 (.sup.1 .DELTA..sup.g). The latter is singlet (molecular) oxygen. These various excited oxygen and nitrogen species also vary greatly in reactive lifetimes.
Prior art electron beam conversion systems were typically designed to fit the composition and flow rate parameters of a single material source. For example, U.S. Pat. No. 4,507,265 describes a power plant effluent gas treatment system in which electron beams convert flowing sulfur oxides and nitrogen oxides into solids and mists for later removal by a dust collector. U.S. Pat. No. 5,015,443, issued May 14, 1991, inventors Ito et al., similarly treats waste gases such as sulfur dioxide and NO with an electron beam where the electron beam is said to form oxygen and hydroxyl radicals from air of the waste gas. This patent also mentions a Japanese patent disclosure said to describe use of atmospheric air introduced into an electron beam reactor to allow the air to be radiated and form ozone and oxygen atoms which is then mixed with a waste gas to oxidize NO.
However, where a conversion facility is dedicated to a single source of material, the reaction chamber and electron beam sources were not designed to accommodate large variations in flow rate or composition. In addition, this prior art technique fails to address the conversion of other toxic materials, such as volatile organic compounds, to relatively benign compounds or elements which can be dealt with by conventional means. Also, these prior art systems have generally been large and immobile and typically have high electric power consumption which is a different type of structure and methodology than that described in the instant application. Further, for some VOC's it is quite difficult to achieve acceptable levels of reduction, even at high applied electron beam dosages.